1. Field of the Invention
This invention relates to a transparent surface, which selectively absorbs, reflects and transmits different wavelengths in a determined fashion. It has particular but not exclusive application in the field of anti-counterfeiting (security) devices.
2. Discussion of Prior Art
In the fight against counterfeiting, there is ever increasing pressure to develop security devices and markings which are difficult to forge i.e. replicate. Moreover it is a requirement that such anti-counterfeiting devices are simple and effective to use without the need for additional, often expensive equipment.
The invention comprises a method of determining whether an article is counterfeit; comprising;
a) providing a textured surface;
b) determining the reflection characteristics of the surface;
c) matching these up with the expected characteristics to determine whether the surface is counterfeit.
In the simplest form of the invention the textured surface comprises of a single (preferably metallic) textured surface.
Preferably the surface is a multilayer consisting of a transparent substrate having at least two thin layers of transparent material deposited on one side thereof, said layers having different refractive indices such that selective wavelengths/colours are transmitted and or reflected The thin multiple layers applied to a transparent substrate provide constructive and destructive interference effects due to multiple reflections at the interfaces between materials.
Preferably the layers are fabricated from metal oxide, metal sulphide or polymeric materials. Individual layers will generally be less than or equal to half a wavelength in thickness when compared to the radiation to be utilised (e.g. for visible light each layer will generally be less than 400 nanometres thick).
The surface may additionally have a coloured or shaded layer applied to the substrate on the opposite of said side to the thin layers.
Such surface may be used as security anti/counterfeit tags, the substrate preferably a transparent plastic material
The invention also consists of a method of determining whether an article is counterfeit comprising:
a) providing such a surface as above;
b) determining its transmission absorption frequencies/colours characteristics;
c) matching these up with the expected characteristics to determine whether the surface is counterfeit.
Step (b) may include a comparison of reflected and/or transmitted spectra at different angles of incidence and/or linear polarisation states of the incident radiation.
Where the surfaces are textured step (b) may further include the detection of changes in the polarisation state of reflected radiation.